Book - School of Science and Technology

Equipment for central stores 141at one end and roller type at the other, stand upon a base which is insulated with hardmaterial in order to minimise heat loss downwards by conduction.Since effective use of storage depends upon minimising disturbance to potential stratificationin the water stored, it is good practice to provide vessels with sparge pipes at bothoutlet and return connections. As shown in Figure 5.11, such pipes should extend over thewhole length of the vessel, except for gaps to permit differential expansion. A rule ofthumb suggests that the area of the holes in each sparge should equate to ten times thediameter thereof and be provided over the whole length in rows arranged to face thecritical direction.The quantity of water stored in cylinders of the size considered is such that, when heatedto storage temperature, the increase in volume is considerable. For instance, when water isheated from 50 C to 120 C the volume is increased by about 4.5%, and when heated to150 C the increase is 8%. It is thus necessary to make provision for this increase byallowing a dead water space below the return sparge pipe, equivalent to about one-eighthof the vessel diameter. It is to the bottom of this space that the feed and expansion pipe isconnected.Where pressure is applied by an elevated cistern, as in Figure 5.12, the syphon loopprevents circulation within the expansion pipe and thus, in theory, the dead water spacestagnates and only cold water is allowed to pass to the high level cistern. In practice, somemixing will have taken place with the `live' content of the vessel and there will also havebeen heat transfer by conduction. For installations where pressure is applied by externalmeans, it is good practice to fit a simple heat exchanger in the feed and expansion pipe,rejecting to waste.It is of interest to note that although most applications for thermal storage cylinders onany substantial scale relate to purpose-designed installation, an alternative version isillustrated in Figure 5.13. This consists of a cylinder which is arranged to retain a spaceabove the level of water fill in order to accommodate the volume of expansion and to providea compressible pressure cushion. Unique, but inherent in the concept, is the introductionof a heat exchanger as separation between the water store and the indirect system. TheAir bottle and ventDeadwaterspaceReturnspargepipeAnti single pipeCirculation syphonExpansion pipe (100 mm min)DrainFigure 5.12 Expansion connection to storage vessel